1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus, wherein a recording sheet, on which a radiation image has been recorded, is scanned with reading light, light radiated out of the recording sheet during the scanning with the reading light is detected by photodetectors, each of which is provided with a photoelectric surface, and an image signal representing the radiation image is obtained from signals, which have been generated by the photodetectors.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected by a photodetector and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
When a recording sheet, such as a stimulable phosphor sheet or X-ray film, on which a radiation image has been recorded, is scanned with the reading light, such as the stimulating rays, light radiated out of the recording sheet (i.e. light emitted by the recording sheet, light having passed through the recording sheet, or light reflected by the recording sheet) is obtained. Ordinarily, the light radiated out of the recording sheet varies over a very wide range from weak light to strong light, and therefore has a very wide range of light amount. In order for even weak radiated light to be detected with a sufficiently high accuracy, it is necessary to employ a photodetector having a sufficiently high photo detection sensitivity (hereinafter often simply referred to as the sensitivity).
The photodetectors, which are ordinarily employed in the radiation image recording and reproducing systems described above, are classified into the photodetectors, such as a photo transistor and a photodiode, which utilize the internal photoelectric effects, and the photodetectors, such as a photomultiplier, which are provided with photoelectric surfaces and utilize the photoelectric emission effects at the photoelectric surfaces. In cases where a photodetector provided with a photoelectric surface, such as a photomultiplier, is employed in the radiation image recording and reproducing systems described above, special measures are generally taken in order to enhance the sensitivity of the photodetector. For example, the photodetector of this type is ordinarily provided with a light guide member for guiding the incident light to the photoelectric surface. In such cases, the light guiding performance of the light guide member is improved, and the efficiency, with which the light is guided to the photoelectric surface, is kept as high as possible. Alternatively, a material, such as a bialkali, which has a high quantum efficiency, e.g. Sb-K-Cs, is employed for the photoelectric surface.
However, recently, it has been found that the photodetector provided with a photoelectric surface, the sensitivity of which has been enhanced in the manner described above, is apt to undergo a saturation phenomenon at the photoelectric surface, i.e. the phenomenon in which the sensitivity decreases for a certain length of time after the photodetector has detected a large amount of light. If the signal obtained from the photodetector undergoing the saturation phenomenon is used, an image signal cannot be obtained accurately.
At present, a photodetector provided with a photoelectric surface, which has a high sensitivity and undergoes little saturation phenomenon at the photoelectric surface, has not yet been known. Therefore, in cases where the photodetector provided with a photoelectric surface is used, it was very difficult to accurately obtain an image signal from a recording sheet, which radiates light in an amount varying over a very wide range from weak light to very strong light.